The present invention relates generally to the field of automated apparatus for handling electronic circuit components and, more particularly, to automated apparatus for use in the art of burning-in circuit components prior to their distribution and use. Still more particularly, the present invention is directed to the art of automated insertion of electronic integrated circuit (IC) packages, in the form of dual in-line packages, into sockets on printed circuit (PC) boards for subsequent burning-in.
According to present practices, IC packages are massproduced and installed in electronic circuits within highly sophisticated, complex and costly equipment. As with many massproduced products, IC packages are prone to failure, in many cases within the first one thousand hours of operation. The complexity of the equipment within which such packages are installed makes post-installation failures highly undesirable. For example, when equipment reaches the final inspection stage of production, before failures are detected, the high level skills required for testing and repair add a significant cost to production expenses. Even more significantly, when the product has been installed in the field and a service technician must make warranty repairs, the costs thereby incurred can have a significant effect on profitability. As a result, manufacturers of electronic equipment are damanding ever greater quality and dependability in commercial grade IC packages.
Quality and dependability is enhanced substantially by detection of those IC packages likely to fail in the first few hours of operation, prior to installation of the packages in electronic equipment. One of the most effective methods for detecting flawed IC packages is referred to as "burn-in". According to burn-in techniques, IC packages are stressed within their physical and electrical limits prior to installation, whereby those packages likely to become early failures in completed equipment can be discovered.
Burn-in involves placing a large number of IC packages on one or more PC boards ("burn-in boards"); placing the burn-in boards with the packages mounted thereon in a chamber whose environment, particularly temperature, is controllable; applying direct current (dc) biases to each package on each board in such a manner as to forward and reverse bias as many of the package's junctions as possible, and/or actively clocking each package to its maximum rated conditions, such application of dc biases, clock signals, and loads being accomplished substantially simultaneously to each package; removing the component boards from the chamber after the IC packages have been subjected to the environmental condition of the chamber and the biases, clock signals, and loads for a designated period of time; and removing the IC packages from the burn-in boards.
The IC packages can be electrically tested either within or outside the burn-in chamber, depending on the sophistication of the particular chamber, by applying a room temperature test of critical dc parameters, e.g., input currents and thresholds, output voltages and currents, and, in the case of digital components, by making a functional test to verify truth table performance. In this way, the packages that fail during burn-in are detected and segregated from those that do not fail. Because the packages that do not fail during the burn-in process have withstood substantial stress, such IC packages possess a high degree of dependability and can be installed in highly complex equipment with reasonable confidence that such IC packages will not fail prematurely.
The burn-in process, however, although successful in reduing the expense of troubleshooting failed electronic equipment, is not itself without expense. Substantial capital expenses are necessary to purchase or construct burn-in chambers, burn-in boards, and test equipment. Personnel must be employed and trained to operate the equipment and to monitor the time-consuming process. In some cases entire businesses have been built around performance of the burn-in process. Use of the burn-in process and, consequently, the success of a business that provides a burn-in service, is dependent upon the cost effectiveness of burning-in the IC packages vis-a-vis not burning-in the packages but instead replacing those that fail after installation and use in the field.
One means for improving the cost effectiveness of the burn-in process is a reduction in labor cost, particularly the labor cost associated with loading and unloading IC packages from sockets on burn-in boards. Consequently, efforts were undertaken to automate, in varying degrees, the loading and unloading stages of the process. The savings in labor cost alone can be substantial. For example, assume that a typical efficient worker may load at an insert rate of 1,000 IC packages per hour (although a lower insert rate would be more probable). The insert rate is the rate at which IC packages are actually placed in sockets on a burn-in board, disregarding the time consumed by any activity other than inserting IC packages in the sockets. The insert rate should be distinguished from the throughput rate, which equals the number of IC packages inserted per unit of time including time-consuming overhead chores, such as removing and replacing burn-in boards and loading IC packages into channels, as well as the actual time required to insert the packages. If a pace of 1,000 packages per hour could be maintained for seven of eight daily working hours, a manual insert rate of 7,000 packages per person per day may be achieved. A well designed automatic loader, operated by the same individual, typically could achieve an insert rate of at least 56,000 packages per eight-hour day.
A savings in the cost of labor is not the sole justification for automating the burn-in board loading process. Consistent insertion force, proper package placement, and controlled insertion pressure minimizes wear and tear on burn-in board sockets and socket contacts. Typical manual insertion involves a rolling motion of the hand, which subjects the socket contacts to large flexure forces. After a period of time, the contacts may become fatigued to the point where they do not spring back adequately to assure a good wiping contact.
Automated burn-in board loaders also may prove less hazardous to the IC packages inserted that their human counterparts. A properly designed loader may improve the quality of the service by reducing human contact with the packages, which contact might damage the packages, and by controlling the insertion force, which may reduce the frequency of bent electrical leads on the packages. Hence, automated burn-in board loaders may potentially reduce the cost of labor, extend the average life of a burn-in board, and reduce damage to the IC packages.
Although published art exists relative to the automated insertion of electronic components directly onto PC boards, such art is not deemed generally pertinent to the art of loading burn-in boards. First, burn-in board loaders insert packages onto sockets, which do not vary from socket to socket; whereas, component loaders insert components directly onto the PC board or into a variety of types of sockets. Second, burn-in boards typically are characterized by a uniform arrangement of sockets across an entire board; whereas, a typical PC board not designed for burn-in use is characterized by an apparently random arrangement of component positions across the board. Finally, the packages inserted on a single burn-in board, or a plurality thereof, are all of the same type, having the same size and other physical characteristics; whereas, the components handled by inserters outside the burn-in art may be any of a variety of types, including resistors, capacitors, IC packages, transistors, and the like, giving rise to a wide assortment of sizes, shapes, and number of leads among the components.
The net result of the aforestated differences is reflected in substantially different requirements for general component insertion as compared to burn-in board loading. On the whole, unless designed for a very narrow application, a general component inserter is complex and is slow. The variations inherent to component inserters as to type of target (i.e., socket or PC board), as to position and orientation of the target, and as to component size and shape have spawned apparatus which is of limited value to the art of burn-in board loaders.
U.S. Pat. No. 3,442,430 to Ackerman et al. discloses an apparatus for inserting IC packages onto a PC board. The apparatus comprises a component support, a component feeding assembly adapted to transfer components one at a time onto the support from a suitable supply of components, and a reciprocal component inserter which is adapted to lift a component from the support, which then rotates out of the path of the inserter, and thereafter inserts the component onto a PC board.
Ackerman et al. does not teach the art of burn-in board loading and, consequently, reveals only a particular mechanical apparatus for delivering a component from a place of storage to a point on a PC board. Ackerman et al. apparently does not disclose means for positioning the apparatus and the target point on the PC board relative to one another. Rapid positioning with high accuracy is essential to the operation of an effective burn-in board loader. In addition, Ackerman et al. discloses but a single insertion mechanism which apparently would operate at a relatively slow pace. The physical bulk of the apparatus prohibits a plurality of such devices operating simultaneously on the same board. Consequently, it is doubtful that implementation of Ackerman et al. or any of the features disclosed therein in a burn-in board loader would yield a cost-effective apparatus.
U.S. Pat. NO. 4,304,514 to Pfaff discloses an apparatus designed particularly for the purpose of loading and unloading burn-in boards. The portion of the apparatus for loading a burn-in board includes a support surface, which receives a gravity-fed supply of circuit packages from a channel, a pressure pad, which is spaced apart from the support surface whereby a circuit package may be slidably received therebetween, and an air cylinder with a shaft which supports the support surface and pressure pad. Actuation of the air cylinder causes the circuit package to be lowered with the support surface to a socket, whereupon the pressure pad forces the circuit package into the socket while the support surface lowers into a slot down the center of the socket so as not to frustrate insertion by the pressure pad. When insertion is completed, the burn-in board is repositioned to slide the support surface from beneath the circuit package and the air cylinder shaft is retracted.
The apparatus of Pfaff, although apparently sound from a functional point of view for use in loading a burn-in board, leaves unresolved certain problems which are evident when a burn-in board loader is operated in an actual working environment. The primary purpose for a burn-in board loader, as pointed out above, is to increase the throughput rate for board loading without a corresponding increase in overhead expense and to improve the quality of the loading process. Yet, certain features of Pfaff, or the lack thereof, tend to diminish the throughput rate which might otherwise be achieved.
It appears that the physical arrangement of the apparatus of Pfaff inhibits the removal and replacement of burn-in boards, so as to redce the speed with which an operator can perform such a task. It further appears that the apparatus has only limited provision for storage of IC packages which are ready to be burned-in, requiring constant attention of the operator to replenish the supply of the packages. The apparatus of Pfaff is capable of loading at most three columns of sockets simultaneously, necessitating the time-consuming process of repositioning of the burn-in board in a lateral as well as longitudinal direction. Because the spacing between the three loader columns of the apparatus is fixed, the apparatus apparently would not be readily adaptable to burn-in boards of a different size. Hence, a different apparatus must be constructed for each size burn-in board used.
Often times fatigue of a socket due to normal wear and tear will necessitate its removal from the burn-in board, leaving an open position on the board. The apparatus of Pfaff makes no provision for detecting such positions. The longitudinal alignment mechanism of Pfaff, including light emitting and detecting devices, appears to be inadequate to align by rows of sockets on a burn-in board where the spacing between rows is very close or is obscured by components, such as capacitors, on the burn-in board. The control system of Pfaff, although not disclosed in any detail, centers around a programmable controller, which permits only limited feedback control and no troubleshooting capability.
Finally, the support surface of Pfaff, inasmuch as it does not span the width between the two rows of electrical leads on the IC package, allows substantial freedom for the package to shift around on the support surface and, hence, to move out of alignment with the socket. This particular feature of Pfaff is critical because, if the IC package is misaligned on the support surface, the electrical leads of the package will not drop into the socket contacts when the package is lowered and consequently the package will be crushed. Thus, the frequency at which the Pfaff apparatus is likely to "crunch," as it is known in the industry, IC packages is unacceptably high.
Hence, it is apparent that the process of burning-in IC packages has become a valued asset to the semiconductor industry and that the success and expansion of the process is directly related to the development of an efficient and reliable burn-in board loader having an optimum throughput rate. It should further be apparent that certain known apparatus do not fully address the requirements of such a device.